1. Field of the Invention
This invention relates generally to systems for controlling electromagnetic devices having solenoids and more particularly to a control circuit having a single boost voltage circuit used to activate a plurality of electromagnetic devices.
2. Description of the Prior Art
Electronic fuel injection control system for internal combustion engines such as diesel engines may include a plurality of electromagnetically actuated fuel injection valves that are sequentially charged with a determinable quantity of fuel during a fuel metering interval and which during a subsequent interval are caused to inject this determinable quantity of fuel into the engine. Such the injector is disclosed in the commonly assigned U.S. Ser. No. 6,949 by Walter et al, now U.S. Pat. No. 4,235,374, entitled "Electronically Controller Diesel Unit Injector" which is expressly incorporated by reference. A characteristic of many electromagnetically actuated devices is that the solenoid associated therewith requires a first high level current to cause the solenoid to activate or pull in a movable member. By utilizing a second substantially lower value of current to the solenoid coil, it is possible to maintain the solenoid (injector) in an open or activated state. Furthermore, it is known in the art that to increase the speed of operation of a solenoid, it is desirable to overdrive the solenoid by applying a large first level of driving current for a limited period of time and to reduce the driving current, to the second or holding level once the solenoid has opened.
As the frequency of activation increases such as when the speed of an engine increases the injector activation and fuel metering periods get closer in time and the metered quantity of fuel must be delivered to injector at an accelerated rate. At some determinable engine operating condition, in order to deliver the metering quantity of fuel to each injector, the metering signals will begin to overlap one another. However, to operate the engine at these increased speeds it has been common practice to simultaneously inject fuel into a number of cylinders or to resort to complicated pulse width variation circuits to operate the system at high speeds.